Semiconductor devices are often manufactured in package form, in which encapsulant materials provide environmental protection to the circuit. The encapsulant materials are exposed to heat generated by semiconductor devices during operation. The encapsulant materials are typically plastics, which are poor thermal dissipators.
Overheating can damage the semiconductor device, disrupting its operation and causing dangerous electrical shorts. In order to minimize the risk of overheating, some packages are provided with a heatsink or heat transfer system that conducts heat away from the semiconductor device to the outer surface of the package. These heat transfer systems generally take the form of metal slugs or strips embedded in the encapsulant material.
The typical heatsink or heat transfer system thus provides internal thermally conductive paths from the die to the outer surface of the package. These paths minimize the risk of overheating of the semiconductor devices as heat generated by the device during operation is conducted away from the device, and reduce the amount of nonconductive packaging through which heat must pass to reach the outer surface of the package.
The conductive paths of a typical heat transfer system are generally formed of thermally conductive metals, such as copper, embedded in the encapsulating material. These metals, though highly conductive, adhere poorly to the plastics commonly used as encapsulant materials. When the encapsulant layer does not adhere smoothly to the surface of the semiconductor device, air or moisture can collect in pockets between the encapsulant and semiconductor device and can damage the device and interfere with its operation.
Providing an anodized layer on the surface of the heatsink material can improve encapsulant adhesion. Mahulikar et al. (U.S. Pat. No. 5,367,196) describe a molded plastic semiconductor package including an aluminum or aluminum alloy heat spreader partially embedded in the molding resin. An anodization layer coating the aluminum or aluminum alloy heat spreader can be applied to improve the adhesion between the heat spreader and the molding epoxy. Mahulikar et al. also suggest alternative coatings which enhance adhesion.
However, the addition of an anodized layer or of additional coatings to improve adhesion is disadvantageous in that it adds complexity and cost to the manufacturing process. A heatsink made of material to which encapsulant material could smoothly adhere, without requiring anodization or additional adhesion layers, is clearly desirable.
The manufacture of a semiconductor package with an internal heatsink can also be costly due to the heatsink materials used. The construction of a heatsink can thus increase the associated costs of package manufacturing. In addition, heat transfer systems can be heavy and bulky, leading to an undesirable increase in the size and weight of the semiconductor package.
Ozmat (U.S. Pat. No. 5,402,004) describes a system for dissipating heat from multiple semiconductor chips disposed on a substrate. The substrate is in turn disposed on a metal matrix. The metal matrix rests on a housing in which an aluminum or copper sponge is secured. The housing contains a fluid inlet and outlet whereby fluid travels through the sponge and withdraws heat from the walls of the sponge cells.
A heat transfer system for use in semiconductor packages that is less complex in construction would be advantageous. A heat transfer system that is efficient in size and in cost is also desirable. In particular, a heat transfer system that uses smaller quantities of expensive metals can be produced less expensively and more efficiently. A heatsink material that provides a surface to which encapsulant materials can more securely adhere is also desired.